Magnetic refrigeration is being considered as an alternative technique to gas compressor technology for cooling and heating based on engineering and economic considerations that indicate that magnetic regenerator refrigerators, in principle, are more efficient than gas cycle refrigerators and thus can yield savings in the cost of operation and conservation of energy.
Magnetic refrigeration utilizes the ability of a magnetic field to affect the magnetic part of a solid material's entropy to reduce it and, therefore, increase the lattice (vibrational) part of the entropy in an isothermal process or the temperature of the solid material in an adiabatic process. When the magnetic field is removed, the change or return of the magnetic entropy of the magnetic solid material reduces the temperature of the material. Thus, magnetic refrigeration is effected by cyclic heat dissipation and heat absorption in the course of adiabatic magnetization and adiabatic demagnetization of the magnetic solid material via application/discontinuance of an external magnetic field. A refrigeration apparatus that exhausts or vents the released heat on one side of the apparatus when the magnetic solid material is magnetized and cools a useful load on another side when the magnetic solid material is demagnetized is known in the magnetic refrigeration art as an active magnetic regenerator magnetic refrigerator (also known by the acronym AMR/MR).
U.S. Pat. No. 5,743,095 describes active magnetic refrigerant materials having general molecular formula Gd5(SixGe1−x)4 that provide a giant magnetocaloric effect for use in magnetic refrigerators when 0≦x≦0.5. Also as described in that patent are alloys for 0.5<x≦1.0 which order magnetically between 295 and 335 K and have useful and large (but not giant) magnetocaloric properties. The giant magnetocaloric effect in the former alloys (x≦0.5) is due to a first order magnetic/structural transition,1 and thus these alloys are useful for cooling applications from just below room temperature (275 K) down to liquid hydrogen temperatures (20 K). The large magnetocaloric effect in the latter alloys (x>0.5) is due to a second order magnetic transformation, making these alloys useful magnetic refrigerants for the high temperature layer of a multilayered active magnetic regenerator of a cooling device for the rejection of heat to the ambient, and also for heat pumps to reach ˜−350 K. The upper temperature limit can be increased from 275 K to 300 K by providing x=0.525 and heat treatment pursuant to this invention as described below such that x values of 0.525≦x≦1.0 provide a large magnetocaloric effect material.
For most magnetic refrigeration and heat pump applications, large amounts (e.g. several hundred grams to hundreds of kilograms) of the magnetocaloric materials per device are needed to obtain sufficient cooling. For example, for a highly efficient magnetic air conditioner about 0.5 kg of magnetic refrigerant could provide a cooling power of 1 kW (kilowatt). Since a typical home requires about 5 kW of cooling power, about 2.5 kg of magnetic refrigerant are needed. For less efficient devices, more magnetic refrigerant material is required. The current process for making the giant and the large magnetocaloric materials Gd5(SixGe1−x)4 involves arc-melting the appropriate amounts of the individual elements (Gd, Si, and Ge), but this technique normally is limited to 50 to 100 gram quantities. Larger quantities can be prepared by arc-melting but generally the resulting ingots are inhomogeneous; i.e. parts of the ingot have excellent magnetocaloric properties much greater than other parts of the ingot having lower magnetocaloric properties, which at best are about the same as the current prototype magnetic refrigerant, Gd, for near room temperature applications. The ingot inhomogeneity is readily understood since, for the Gd5(SixGe1−x)4 alloys, the exact Gd to (Si+Ge) ratio 5:4 has been found to be critical. For example, small deviations from the 5:4 ratio have been found to lead to the appearance of the Gd(SixGe1−x), 1:1, or Gd5(SixGe1−x)3, 5:3, phases, and therefore, to significantly reduced magnetic refrigerant cooling capacity.
These very same alloys, which exhibit the giant magnetocaloric effect, also exhibit an extremely large magnetostriction and also a large magnetoresistance when undergoing the first order transformation. Based on crystallographic data, Gd5(Si2Ge2) has a reversible linear colossal magnetostriction of ˜10,000 parts per million (ppm) along the [100] axis, or a volumetric colossal magnetostriction of ˜4500 ppm. In comparison, the magnetostriction of Terfenol-D [(Tb0.7Dy0.3)Fe2] is ˜1200 ppm. Since the colossal magnetostriction is due to the first order phase transition it is expected to occur in all Gd5(SixGe1−x)4 alloys for 0≦x≦0.56. Thus the method of this invention described herein for the production of the giant magnetocaloric materials also applies for producing the colossal magnetostrictive alloys, which are useful as actuators, positioning devices, etc. controlled by the change of the magnetic field and also for magnetoelastic sensors to detect stresses.
Measurements of the electrical resistance of Gd5(SixGe1−x)4 alloys for 0.24≦x≦0.525 as a function of temperature and magnetic field show that there is a large (˜25%) change in the resistance at the first order phase transformation when induced by a magnetic field above their respective ordering temperatures. The sign of the change is positive for x=0.375 and negative for x=0.5. Such large changes have been observed in artificial, non-rare earth, magnetic multilayered materials, and have been labeled as “giant” magnetoresistors. There are a number of applications in the electronics field for giant magnetoresistance materials, including read heads in magnetic recording devices and sensors.
Furthermore, the Si to Ge ratio is also important since the magnetic ordering (Curie) temperature (TC) is strongly dependent on the relative amounts of these two elements. For example, for x greater than 0.525 for the Gd5(SixGe1−x)4 material, the giant magnetocaloric, colossal magnetostriction, and giant magnetoresistance effects are not observed; and for x greater than or equal to 0 and less than or equal to 0.525, the Curie temperature varies almost linearly with x from approximately 20 K at x=0 to approximately 300 K at x=0.525, and the material exhibits the giant magnetocaloric, colossal magentostrictive, and giant magnetoresistance effects.
Arc-melting 100 gram quantities at a time is labor intensive and thus an extremely expensive operation. Furthermore, use of commercially pure Gd (having more than 0.1 wt. % interstitial and other impurities) instead of high purity Gd (less than 0.1 wt. % impurities) to prepare the Gd5(SixGe1−x)4 material for 0≦x≦0.525 by arc-melting has led to only average magnetocaloric properties and not the giant magnetocaloric properties desired. The magnetocaloric properties of Gd5(SixGe1−x)4 for 0.525≦x≦1.0, however, are not nearly as sensitive to the impurities in the Gd metal used to prepare the alloys by arc-melting.
Copending Ser. No. 09/793,822, now U.S. Patent 6,589,366, describes a method of making relatively larger quantities of the Gd5(SixGe1−x)4 material for 0≦x≦1.0 using commercially pure Gd, Si and Ge as starting charge components in a more cost effective manner.
The present invention provides a heat treatment for Gd5(SixGe1−x)4 and other alloys to provide heat treated material having improved magnetocaloric and other magnetothermal properties that are better than those of Gd metal at and near room temperature.